This invention relates to the production of a mineral melt by burning combustible material in the presence of inorganic particulate material and thereby forming a melt. The melt can then be fiberised to form mineral fibres or used in other industrial processes.
Traditionally, the normal way of producing a melt for slag, stone or rock fibres has been by means of a shaft furnace in which a self-supporting stack of inorganic particulate material is heated by combustion of combustible material in the furnace. The stack gradually melts and is replenished from the top, with melt draining down the stack and out from the bottom of the furnace. The normal furnace for this purpose is a cupola furnace.
It is necessary for the stack to be self-supporting and permeable to the combustion gases, which are generally generated by combustion of carbonaceous material in the stack. It is therefore necessary that everything in the stack is relatively coarse (in order that the stack is permeable) and has high physical strength and does not collapse until combustion or melting is well advanced. In practice this means that the carbonaceous material is coke and the particulate material is either coarsely crushed rock, stone or slag or is in the form of briquettes formed from fine particulate material.
Accordingly, if the material which is available is only available in finely divided form, it is necessary to incur the expense and inconvenience of forming it into briquettes. Briquetting usually uses sulphur-containing materials as binder, such as Portland cement with gypsum, and this means that the effluent is liable to have a high sulphur content, which has to be treated.
The cupola or other stack furnace system also has the disadvantage that conditions in the furnace always tend to be sufficiently reducing that some of the iron is reduced to metallic iron. This necessitates separating metallic iron from the melt, reduces wool production, leads to the provision of iron waste and also tends to incur the risk of corrosion in the zone containing iron and slag.
Another disadvantage is that the process does not have high thermal efficiency.
Despite these disadvantages, the process using a cupola or other stack furnace has been widely adopted throughout the world for the manufacture of rock, stone or slag fibres.
An alternative and entirely different system for the production of a mineral melt that avoids or reduces the disadvantages of the cupola system is disclosed in our earlier publication WO 03/002469. This system involves suspending powdered coal, or other fuel, in preheated combustion air and combusting the suspended fuel in the presence of suspended particulate mineral material in a circulating combustion chamber, i.e., a combustion chamber in which the suspended particulate materials and air circulate in a system which is or approaches a cyclone circulation system. This is commonly referred to as a cyclone furnace.
The suspension of coal in preheated air, and the particulate mineral material, are introduced through the top or close to the top of the combustion chamber. Within the combustion chamber, combustion of the particulate coal occurs and the particulate material is converted to melt. The melt and particulate material that is not yet melted is thrown onto the walls of the chamber by the circulating gases and will flow down the chamber.
In WO03/002469, the combustion chamber preferably leads downwards into a large settling tank which has a considerably enhanced volume. There may be a gas burner or other means for supplying extra energy to the settling tank to raise the temperature of the exhaust gases. The burner is positioned towards the top of the settling tank. The exhaust gases which are free of melt are taken from the settling tank or the combustion chamber up through a duct at the top of the chamber.
In order to increase the energy efficiency of the cyclone furnace in WO03/002469, the exhaust gases, which leave the circulating chamber at a temperature in the range of 1400 to 1700° C. are used to preheat the particulate material so as to use rather than waste this heat energy. This step can be carried out under conditions which reduce nitrogen oxides (NOx) which reduces the environmental effects of the exhaust gases. The exhaust gases then pass through another heat exchanger by which there is indirect heat exchange with the combustion air.
The cyclone furnace has significant advantages compared to cupola or other stack furnaces. With respect to fuel, it avoids the need for briquetting fine particles and a wide range of fuels can be used including, for example, plastic. Using a melting cyclone furnace eliminates the risk of reduction of the ores to iron and releases exhaust gases which are environmentally acceptable. The flexibility in melt capacity is much better than with a cupola furnace meaning that production can easily and quickly be switched, from, for example, 40% to 100% of total capacity so the time taken to respond to changing demands is greatly reduced. Furthermore, melting in a cyclone furnace is much quicker than is the case for a cupola furnace and is in the order of minutes, rather than in the order of hours.
Hence, using a melting cyclone furnace system is economically and environmentally desirable and the system disclosed in WO 03/002469 works well. There is, however, room for improvement in the process.
In particular, although several steps are taken to recycle the large amount of energy used in producing the melt, there is inevitably a large amount of energy that is lost due to the large volume of the settling tank and the high volume of combustion air which is used. It is desirable to increase the energy efficiency of the system further.
WO03/002469 suggests a second embodiment shown in FIG. 2 in which the settling tank is replaced by a relatively small collection zone at the base of the combustion chamber. Such systems would lead to increased energy efficiency due to the reduced volume of the apparatus through which energy is lost. However, the inventors have found that in this system the melt quality is reduced.
In processes for making mineral fibres, such as that in WO03/002469, the purity of the melt is extremely important as it has a direct effect on the quality of the mineral fibres produced.
U.S. Pat. No. 4,365,984 is also concerned with producing mineral wool using a melting cyclone furnace and involves feeding a particulate waste material containing inorganic non-combustible and organic combustible components into combustion air. As in WO03/002469, the system includes a large collection zone. The exhaust gases are cooled in a heat exchanger and the waste heat is said to be suitable for drying coal or other waste or can be used in other processes or for heating purposes. The fuel is coal which is pulverised. Coarse fuel particles are said not to burn completely and hence become entrapped in the molten slag.
U.S. Pat. No. 4,544,394 concerns a method of melting glass in a vortex reactor. The process is designed to use liquid or gaseous fuels. The fuels undergo preliminary combustion in a gas or oil fired suspension preheater before introduction into the vortex where they combust further.
U.S. Pat. No. 6,047,566 concerns a method for melting recycled silicate materials. Due to the need to oxidise organic materials that are present as impurities in the recycled silicate materials, a long dwelling time inside the combustion chamber is needed. In this document a pre-burner for combustion of the fuel and combustion air is provided externally of the combustion chamber.
U.S. Pat. No. 4,957,527 is concerned with making glass in a cyclone melt reactor. This document mentions that using ash containing fuels such as coal has in the past been difficult because of the ash contamination in the glass. The apparatus makes use of large preheated suspension chamber in which the mineral materials are melted prior to introduction into the cyclone chamber where separation and dispersion of the materials occurs.
Melting cyclones are also known in other fields, particularly the field of pyrometallurgic processes (such as in U.S. Pat. Nos. 4,566,903 and 5,282,883). In such processes, the end product is a molten metal and any molten mineral material that is present is a waste material. Therefore, the quality of the mineral melt is unimportant in such processes.
In US 2005/0039654, a cyclone chamber is used to combust fuel to generate energy for use for other purposes. Mineral material is not added to the system as the purpose is not to make a melt, but the fuel that can be used can be so-called “slagging coal” which contains some mineral materials that are not combustible but melt to form a slag when the coal is combusted. This publication is concerned with the selective use of oxygen enrichment at a point in the barrel of the cyclone combuster to maintain the slag in a molten form, to minimise NOx emissions and to minimise the escape of fine coal particles in the barrel. Air (referred to as a first or primary oxidant having an oxygen concentration of about 21% by volume) is introduced into the burner with the fuel. A second oxidant stream which has a concentration greater than the first can be introduced either into a region adjacent to the coal, or into the barrel. The second oxidant mixes with a portion of the first oxidant to give a region of mixed oxidant which is said to contain less than about 31% oxygen by volume (so the oxygen level of the total oxidant i.e. combustion gas is much lower than 31%). There is no suggestion in this publication to increase the levels of oxygen in both the primary and secondary air streams.
Hence, in known processes for making mineral fibres using a cyclone furnace, it has generally been necessary to use more than one chamber to carry out the process of combustion and melt collection.
The present invention is concerned with increasing the energy efficiency of known methods for making mineral fibres using the melting cyclone furnace system while ensuring that the quality of the melt is high.